The present invention relates to rigid polyurethane foams with desiccating properties and to hybrid polyurethane-polyimide foams of similar character.
It is known that molded desiccant articles can be made from various desiccant materials in combination with binders. For example, molded blocks comprising a desiccant and an epoxy resin are disclosed in, e.g., U.S. Pat. No. 3,545,622 to Sakhnovsky et al. Such articles generally have a density of about 1 g/cc. Higher density structural desiccants can be made from molecular sieves, glass frit, and bentonite, as reported in, e.g., U.S. Pat. No. 3,235,089 to Burroughs.
Low density molded desiccant articles can also be made as a syntactic foam by blending a thermosetting resin, microspheres, and molecular sieve powder, then molding and curing the mixture. The resin used may be a polybenzimidazole, a polyphenylene, a phenolic, or a polyimide resin. The resulting articles, as disclosed by Lula et al. in U.S. Pat. No. 4,447,565, have densities of about 0.2 to 0.9 g/cc, moisture capacities of 1 to 12% by weight, and can serve as light weight structural supports. However, these excellent foam products are made with rather expensive resins, e.g. polyimides, by a process requiring relatively high temperature and isostatic pressures.
As to polyurethanes, there are many disclosures of such compositions with inorganic additives to affect various properties. For example, Illger et al. U.S. Pat. No. (4,288,559) add alumino-silicates to a polyurethane foam to improve structural stability and fire resistance. Alumina hydrate or silica is also used by Kehr et al. U.S. Pat. No. (3,897,372) for fire retardance. Gruber U.S. Pat. Nos. (3,325,844) and Weinberg (3,271,352), on the other hand, add zeolites to polyurethane lacquer and coating material to remove water from the reactants so that clear flawless film products are obtained. These compositions are neither foams nor desiccants nor do they possess any structural capabilities. Deffeyes U.S. Pat. No. (4,036,360) also adds at least 20% of a particulate desiccant, which may be zeolite, to a prepolymerized polyurethane which is then formed into strips to be inserted in packages for moisture adsorption. Heretofore, as Deffeyes notes, high desiccant-content resins were fragile.